Metal-halide based perovskite solar cells have rapidly emerged as a promising alternative to traditional inorganic and thin-film photovoltaics. Metal-halide perovskite solar cells have demonstrated dramatic improvements in efficiencies over the past six years, rising from about 3% in 2009 to above 20% in 2015. These hybrid organic/inorganic photovoltaics embody many advantages of inexpensive solution-processed solar cells (e.g. bulk heterojunction organic photovoltaic (PV) devices, dye-sensitized cells, etc.), but at greater power conversion efficiencies that compare favorably with established inorganic technologies, such as CdTe and polycrystalline silicon. Despite the rapid rise in efficiency, a number of fundamental operational principles of perovskite solar cells are still poorly understood. Particularly important are the mechanisms and rates by which photogenerated charge carriers (1) recombine within neat perovskite films (either within the bulk or through traps), (2) diffuse to interfaces with electron and hole transport layers (ETL and HTL), (3) are transferred across these interfaces, and (4) recombine following extraction by the ETL and HTL. The rates for these competing processes help to determine the ultimate efficiency of devices. Rapid diffusion and interfacial charge transfer compete with recombination within the perovskite layer and aid in charge extraction by the ETL and HTL, enhancing photocurrent. Suppressing back-transfer from the ETL and HTL into the alkyl ammonium metal halide perovskite layer should result in long-lived charge separation and reduced recombination losses to the open circuit voltage and fill factor.